You can fly most things with a CPL, as an FO - provided that you have a rating for it. Most pilots start off this way, as they do not have the hours for an ATPL, they will have a "frozen" ATPL. Essentially that is just a CPL with ATPL knowledge, i.e they have taken all the ATPL exams. When they have the requisite hours for an ATPL they upgrade their licence. You would in practice find that many employers will not let you fly long haul without an ATPL as they like you to be able to act as commander when the Captain is in the bunk, but that is down to the individual company. Quite what the rules are regarding being a Captain are with just a CPL I'm not sure, but then most people don't start that way. Hope that helps. Anyone else with greater knowledge please feel free to correct the above.